This paper discusses the monitoring of a project demonstration of a 20 kW grid-connected photovoltaic system(GCPVS) in Phitsanulok, Thailand. The GCpVS was installed in order to evaluate the grid-connected photovoltaic technology, which operates under the real condition. The aim of the evaluation is to identify which type of solar modules will be appropriate for grid-connected power generation in Thailand's northern climatic conditions. The GCPVS consists of three sub-arrays (monocrystalline sub-arrays, CdTe thin-film sub-array and a-Si thin-film sub-arrays), which was each designed to generate equal electrical power (~7.2 kW), D.C. control cabinet ,2.5 kW battery back up, charging controller and three grid-connected type single phase inverters. Technical aspects of each sub-array in the GCPVS have been investigated in terms of sub-array DC voltage and ampere, AC voltage and ampere as well as total power yield. The efficiency of each sub-array was also analyzed, and taken into consideration. Since the operational data have been seriously monitored in August 2003, results show that the monocrystalline sub-array could generate total power yield up to 2.97 kWh/kWp/day whereas the total power yield of a-Si thin-film sub-array was about 0.84 kWh/kWp/day. But the data of CdTe thin-film was not available.